Switch box

ABSTRACT

A switch box includes a relay transistor circuit connecting a pair of batteries in parallel, and an breaker circuit that breaks the relay transistor circuit when a sign of a potential difference between a potential of at least one of the pair of batteries connected to the relay transistor circuit and a predetermined reference potential is reversed with respect to the sign of the potential difference in which the batteries are correctly connected.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application claims priority to and incorporates by referencethe entire contents of Japanese Patent Application No. 2014-226179 filedin Japan on Nov. 6, 2014.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a switch box.

2. Description of the Related Art

More devices in more fields have been electronized recently, and aplurality of secondary batteries are used as a power supply to satisfythe demand for a sufficient amount of electricity.

For example, provided to some automobiles is an in-vehicle power supplysystem using a lithium-ion battery as a sub-battery for supplying powerto an in-vehicle equipment, such as an audio device, in addition to alead storage battery serving as a main battery for driving the startermotor for starting the engine.

In addition to the demand for electricity, some other reasons for theuse of the in-vehicle power supply system include the demands forimproving the lifetime of devices and avoiding cost increase.

Lead storage batteries as the main battery, which are less expensivethan the lithium-ion batteries as a sub-battery, are less durableagainst frequent charging/discharging. In particular, the lead storagebattery deteriorates quickly due to frequent charging/discharging in,for example, vehicles with idling stop function, which stops the enginewhen the vehicles are stopping, or vehicles generating and storingelectric power with an alternator using the regenerative energy of thevehicles.

Therefore, the less expensive lead storage battery is used incombination with the lithium-ion battery, which is more durable againstfrequent charging/discharging. The lithium-ion battery is preferentiallyused in operations requiring frequent charging/discharging, such as anoperation of supplying power for a load during idling stop andregenerative charging, so that the lead storage battery deterioratesless. The lead storage battery is then used to store power for backuppower supply for a long time period, for example, so as to avoid costincrease due to use of the lithium-ion battery, which has a smallcapacity. A related-art example is disclosed in Japanese PatentApplication Laid-open No 2009-166769. In such an in-vehicle power supplysystem, when one of the main battery and the sub-battery is connectedwith its polarities reversed after a replacement, for example, a currentmay flow from a battery on the higher potential side to the otherbattery on the lower potential side (hereinafter, such a current isreferred to as a reverse connection current).

This creates a need for a switch box that allows building of anin-vehicle power supply system in which such a reverse connectioncurrent flow can be prevented reliably.

SUMMARY OF THE INVENTION

The present invention is made in consideration of the above, and anobject of the present invention is to provide a switch box capable ofpreventing the reverse connection current reliably when a battery isconnected reversely.

In order to achieve the above mentioned object, a switch box accordingto one aspect of the present invention includes a relay transistorcircuit configured to connect a pair of batteries in parallel; and abreaker circuit configured to break the relay transistor circuit when asign of a potential difference between a potential of at least one ofthe pair of batteries connected to the relay transistor circuit and apredetermined reference potential is reversed with respect to the signof the potential difference in which the batteries are correctlyconnected.

According to another aspect of the present invention, in the switch box,the relay transistor circuit may include a pair of N-channel MOSFETshaving source terminals connected back to back, and the breaker circuitmay include a short-circuiting circuit that short-circuits the sourceterminal and a gate terminal of each of the N-channel MOSFETs.

According to still another aspect of the present invention, in theswitch box, the short-circuiting circuit may include a short-circuitingN-channel MOSFET connected serially between the source terminal and thegate terminal of each of the N-channel MOSFETs, and having a gateterminal set at the reference potential.

According to still another aspect of the present invention, the switchbox may further include an off circuit configured to apply a high levelvoltage to the gate terminal of the short-circuiting N-channel MOSFET soas to switch off the N-channel MOSFETs included in the relay transistorcircuit, based on an all-off control signal from outside.

According to still another aspect of the present invention, in theswitch box, the reference potential may be set to a ground potential.

According to still another aspect of the present invention, in theswitch box, the pair of N-channel MOSFETs may have gate terminalscommonly connected, the relay transistor circuit may include a pluralityof pairs of relay transistor circuits provided in parallel between thepair of batteries, and the short-circuiting circuit may be commonlyconnected to the N-channel MOSFETs included in each of the relaytransistor circuits, and may short-circuit the source terminal and thegate terminal of each of the N-channel MOSFETs simultaneously.

The above and other objects, features, advantages and technical andindustrial significance of this invention will be better understood byreading the following detailed description of presently preferredembodiments of the invention, when considered in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of the configuration of anin-vehicle power supply system including a pair of in-vehicle batteries;

FIG. 2 is a schematic block diagram of the configuration of a switchbox; and

FIG. 3 is a detailed diagram for explaining an exemplary circuit of theswitch box.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of the present invention will be explained withreference to the accompanying drawings.

FIG. 1 is a schematic block diagram of the configuration of anin-vehicle power supply system including a pair of in-vehicle batteries.

This in-vehicle power supply system 10 includes a main battery 11 thatis a lead storage battery, a sub-battery 12 that is a lead storagebattery, an in-vehicle controller 13 provided as, for example, anelectronic control unit (ECU), and a switch box 14 including solid-staterelays (SSRs) provided as relay transistor circuits performingoperations such as parallel connection and disconnection of the mainbattery 11 and the sub-battery 12 under the control of the in-vehiclecontroller 13.

With such a configuration, the in-vehicle controller 13 in a test modeperforms control to switch on or off each of a plurality of field-effecttransistors (FETs) included in the switch box 14, and uses a resultanttest voltage VTST to detect a short failure and an open failure in eachof the FETs.

The in-vehicle controller 13 in a normal operation mode performs controlto adjust a state of charge (SOC) of the main battery 11 or thesub-battery 12 to be within an appropriate range to prevent the mainbattery 11 and the sub-battery 12 from deteriorating quickly due toovercharge or overdischarge.

In other words, when the SOC of the main battery 11 falls below theappropriate range, the in-vehicle controller 13 promotes charging of themain battery 11 by setting a high voltage to the voltage of generatedpower that is adjusted by a regulator. When the SOC of the main battery11 exceeds the appropriate range, the in-vehicle controller 13 promotesdischarge of the main battery 11 by setting a low voltage to the voltageof generated power that is adjusted by the regulator.

It is normal for the open-circuit voltage of the main battery 11 to bedifferent from that of the sub-battery 12. Therefore, if the mainbattery 11 and the sub-battery 12 are electrically connected, the setvoltage of generation power may be adversely changed to keep the SOC ofthe main battery 11 within the appropriate range, which may causeovercharge or overdischarge of the sub-battery 12.

Therefore, when it is desirable to charge the main battery 11 but notthe sub-battery 12, or it is not desirable to allow the power of thesub-battery 12 to be discharged into the main battery 11, the in-vehiclecontroller 13 instructs the switch box 14 to disconnect the main battery11 from the sub-battery 12, or to keep these batteries disconnected.

FIG. 2 is a schematic block diagram of the configuration of the switchbox.

The switch box 14 includes a plurality of (three, in FIG. 2) relaytransistor circuits 21-1 to 21-3 that are connected in parallel, asimultaneous ON circuit 22 for switching all of the relay transistorcircuits 21-1 to 21-3 ON simultaneously based on an all-ON controlsignal C_(ALL) _(_) _(ON) received from the in-vehicle controller 13, asimultaneous OFF circuit 23 for switching all of the relay transistorcircuits 21-1 to 21-3 OFF simultaneously based on an all-OFF controlsignal C_(ALL) _(_) _(OFF) received from the in-vehicle controller 13,individual driving circuits 24-1 to 24-6 that are provided in pairs forthe respective relay transistor circuits, a main voltage detectingcircuit 25 that detects a voltage VMB of the main battery 11, asub-voltage detecting circuit 26 that detects a voltage VSB of thesub-battery 12, and a test voltage detecting circuit 27 that detects thetest voltage VTST in the test of the relay transistor circuits 21-1 to21-3. The individual driving circuits 24-1 to 24-6 drive respective sixN-channel metal-oxide semiconductor field-effect transistors (MOSFETs)included in the relay transistor circuits 21-1 to 21-3, which will beexplained later, based on individual ON/OFF control signals C_(ON) _(_)_(OFF11), C_(ON) _(_) _(OFF12), C_(ON) _(_) _(OFF21), C_(ON) _(_)_(OFF22), C_(ONOFF) _(_) ₃₁, and C_(ON) _(_) _(OFF32), respectively,received from the in-vehicle controller.

The plurality of relay transistor circuits are provided in this exampleso as to distribute the current into these circuits and achieve a highersubstantial electric capacity.

FIG. 3 is a detailed diagram for explaining an exemplary circuit of theswitch box.

As illustrated in FIG. 3, each of the relay transistor circuits 21-1 to21-3 includes N-channel MOSFETs 31-1 and 31-2 that are provided in pairand the source terminals of which are connected back to back. Constantvoltage circuits CV for preventing a reverse-current flow are providedbetween the source terminal and the gate terminal of the N-channelMOSFET 31-1, and between the source terminal and the gate terminal ofthe N-channel MOSFET 31-2. In each of the constant voltage circuits CV,a pair of Zener diodes having their anodes connected to each otherprevents a reverse current flow. The constant voltage circuits CV applya predetermined constant voltage between the source terminal and thegate terminal of the N-channel MOSFET 31-1, or between these terminalsof the N-channel MOSFET 31-2.

As illustrated in FIG. 3, the simultaneous ON circuit 22 includes a PNPtransistor 33, a booster circuit 34, and an NPN transistor 36. Thecollector terminal of the PNP transistor 33 is connected to the gateterminals of the N-channel MOSFETs 31-1 and 31-2 provided in pair andincluded in the relay transistors, via a current restricting resistor32. The booster circuit 34 is connected to the emitter terminal of thePNP transistor 33, and configured to apply an “H” (high) level voltage.The base terminal of the NPN transistor 36 receives an input of theall-ON control signal C_(ALL) _(_) _(ON) from the in-vehicle controller.The collector terminal of the NPN transistor 36 is connected to the baseterminal of the PNP transistor 33 via a pull-down resistor 35. Theemitter terminal of the NPN transistor 36 is grounded. The NPNtransistor 36 drives the PNP transistor 33 based on the all-ON controlsignal C_(ALL) _(_) _(ON).

In the configuration described above, a constant voltage circuit CV2 isprovided between the base terminal and the emitter terminal of the PNPtransistor 33. The constant voltage circuit CV2 includes a Zener diodeconnected in parallel with a resistor, for keeping the voltage betweenthe base terminal and the emitter terminal to a constant voltage whilethe PNP transistor 33 is ON state.

As illustrated in FIG. 3, the simultaneous OFF circuit 23 includes anN-channel MOSFET 37 for off-controlling, a pull-down resistor 38, a PNPtransistor 39, and an NPN transistor 41. The source terminal of theN-channel MOSFET 37 is connected to the source terminals of theN-channel MOSFETs 31-1 and 31-2 in pair, and the drain terminal of theN-channel MOSFET 37 is connected to the gate terminals of the N-channelMOSFETs 31-1 and 31-2 in pair. The N-channel MOSFET 37 is configured tobe switched ON to produce a short circuit between the source terminaland the gate terminal of each of these N-channel MOSFETs. The pull-downresistor 38 connects the gate terminal of the N-channel MOSFET 37 to areference potential Vref (=0 volts). The PNP transistor 39 has itscollector terminal connected to the connection point between theN-channel MOSFET 37 and the pull-down resistor 38, and has its emitterterminal connected to the booster circuit 34. The base terminal of theNPN transistor 41 receives an input of the all-OFF control signalC_(ALL) _(_) _(OFF) from the in-vehicle controller. The collectorterminal of the NPN transistor 41 is connected to the base terminal ofthe PNP transistor 39 via a pull-down resistor 40, and the emitterterminal of the NPN transistor 41 is grounded. The NPN transistor 41 isconfigured to drive the PNP transistor 39 based on the all-OFF controlsignal C_(ALL) _(_) _(OFF).

In such a configuration, the N-channel MOSFET 37 and the pull-downresistor 38 serve as a breaker circuit that breaks the relay transistorcircuits 21-1 to 21-3. The N-channel MOSFET 37 serves asshort-circuiting circuit, that is, serves as a short-circuitingN-channel MOSFET for short-circuiting the source terminal and the gateterminal of each of the N-channel MOSFETs 31-1 and 31-2 included in therelay transistor circuits 21-1 to 21-3. With this configuration, theshort-circuiting circuit is implemented solely with the N-channel MOSFET37, thereby achieving a simple circuit configuration, and reliable breakof the relay transistor circuits 21-1 to 21-3.

In addition, a constant voltage circuit CV1 for preventing a reversecurrent flow is provided between the source terminal and the gateterminal of the N-channel MOSFET 37. The constant voltage circuit CV1includes a pair of Zener diodes the anodes of which are connected andthat prevent a reverse current flow. The constant voltage circuit CV1applies a predetermined constant voltage between the source terminal andthe gate terminal of the N-channel MOSFET 37. A constant voltage circuitCV3 is provided between the base terminal and the emitter terminal ofthe PNP transistor 39 for keeping the voltage between the base terminaland the emitter terminal to the constant voltage while the PNPtransistor 39 is ON state. The constant voltage circuit CV3 includes aZener diode connected in parallel with a resistor.

The individual driving circuits 24-1 to 24-6 are configured to drive theN-channel MOSFETs included in the corresponding relay transistorcircuits, and all have the same configuration. In the following, theindividual driving circuit 24-1 will be explained as an example withreference to FIG. 3.

The individual driving circuit 24-1 includes a PNP transistor 52, a biascircuit 53, and an NPN transistor 54, as illustrated in FIG. 3.

In such a configuration, the collector terminals of all of the PNPtransistors 52 in the respective individual driving circuits 24-1 to24-6 are commonly connected to the collector terminal of an NPNtransistor 56 via a pull-down resistor 55.

The base terminal of the NPN transistor 56 receives an input of asneak-current preventing control signal C_(STP) from the in-vehiclecontroller 13. The NFN transistor 56 is switched ON during theindividual ON/OFF operation. The gate terminals of the N-channel MOSFETs31-1 and 31-2 that are not to be controlled are grounded state (“L”(low) level) so that such N-channel MOSFETs 31-1 and 31-2 are notoperated by the sneak current of the power for controlling the N-channelMOSFET 31-1 (or the N-channel MOSFET 31-2) being controlled. In order toprevent such a situation, the pull-down resistor 55 and the NPNtransistor 56 serve as a sneak current preventing circuit 28.

These individual driving circuits 24-1 to 24-6 and the sneak currentpreventing circuit 28 are used in testing the operations of theindividual N-channel MOSFETs 31-1 and 31-2 included in the relaytransistor circuits 21-1 to 21-3.

Operations according to the embodiment will be explained.

[1] All-ON Operation

To begin with, an all-ON operation in which all of the N-channel MOSFETs31-1 and 31-2 included in the relay transistor circuits 21-1 to 21-3 areswitched ON will be explained.

At an initial state, all of the N-channel MOSFETs 31-1 and 31-2 includedin the relay transistor circuits 21-1 to 21-3 are all OFF state. Thesource terminal of each of the N-channel MOSFETs 31-1 and the sourceterminal of the corresponding N-channel MOSFET 31-2 are connected backto back, and because their parasitic diodes are oriented in oppositedirections, the main battery 11 and the sub-battery 12 are notconnected.

The in-vehicle controller 13 then sets the all-ON control signal C_(ALL)_(_) _(ON) to the “H” level.

This signal causes the NPN transistor 36 to be ON, and the pull-downresistor 35 brings the potential at the base terminal of the PNPtransistor 33 to the ground level, and the PNP transistor 33 is switchedON.

As a result of this operation, the “H” level voltage (=20 volts) of thebooster circuit 34 is applied to the gate terminal of the N-channelMOSFET 31-1 and the gate terminal of the N-channel MOSFET 31-2 includedin the relay transistor circuits 21-1 to 21-3, thereby switching all ofthe N-channel MOSFETs 31-1 and 31-2 to be ON (closed state).

The main battery 11 and the sub-battery 12 are thus connected inparallel via the relay transistor circuits 21-1 to 21-3, and supplypower integrally.

[2] All-OFF Operation

An all-OFF operation in which all of the N-channel MOSFETs 31-1 and 31-2included in the relay transistor circuits 21-1 to 21-3 are switched OFFwill be explained.

It is assumed that, at an initial state, the N-channel MOSFETs 31-1 and31-2 included in the relay transistor circuits 21-1 to 21-3 are all ONstate.

The in-vehicle controller 13 sets the all-ON control signal C_(ALL) _(_)_(ON) to the “L” level, switching the NPN transistor 36 and the PNPtransistor 33 OFF thereby. The in-vehicle controller 13 then sets theall-OFF control signal C_(ALL) _(_) _(OFF) to the “H” level.

This signal causes the NPN transistor 41 to be ON. The pull-downresistor 4C brings the potential of the base terminal of the PNPtransistor 39 to the ground level, and the PNP transistor 39 is switchedON.

As a result of this operation, the “H” level voltage of the boostercircuit 34 is applied to the gate terminal of the N-channel MOSFET 37.

The N-channel MOSFET 37 is then switched ON, and the potential of thesource terminals and the gate terminals of all of the N-channel MOSFETs31-1 and 31-2 included in the relay transistor circuits 21-1 to 21-3 arebrought to the same level, and all of the N-channel MOSFETs 31-1 and31-2 are switched OFF.

In this manner, the parallel connection between the main battery 11 andthe sub-battery is broken by the relay transistor circuits 21-1 to 21-3.

[3] Operation when Battery is Connected with Polarity Reversed

An operation with a battery connected with its polarity reversed will beexplained.

[3.1] Operation with Reversely Connected Main Battery

To begin with, at an initial state, it is assumed that the N-channelMOSFETs 31-1 and 31-2 included in the relay transistor circuits 21-1 to21-3 are all OFF state, and the main battery 11 and the sub-battery 12are disconnected.

In such a configuration, because the cathode terminal of a diode D11 inthe main battery 11 is connected with the cathode terminal of a diodeD12 in the sub-battery 12, as illustrated in FIG. 3, no current willflow from the sub-battery 12 being the high-potential side (e.g., +12volts) into the main battery 11 being the low-potential side (e.g., −12volts) unless the N-channel MOSFETs 31-1 and 31-2 are switched ON(closed state).

The potentials of the gate terminals of the N-channel. MOSFETs 31-1 and31-2 are at the ground level.

When the main battery 11 is reversely connected, the source terminals ofthe N-channel MOSFETs 31-1 and 31-2 are brought to the potential equalto the sum of the drop voltage Vf of the parasitic diode of theN-channel MOSFET 31-1 and the potential of the main battery 11 (e.g.,−12+Vf [v]).

As a result, the N-channel MOSFETs 31-1 and 31-2 shift to the ON state(closed state). In other words, some current may flow from thesub-battery 12 into the main battery 11 (or from the main battery 11into the sub-battery 12).

The potential of the gate terminal of the N-channel MOSFET 37 is at thereference potential Vref (=0 volts: ground potential) due to thepresence of the pull-down resistor 38. This ensures that, when at leastone of the main battery 11 and the sub-battery 12 is reverselyconnected, the sign of the difference between the potential of thereversely connected battery and the predetermined reference potentialVref is reversed with respect to the difference achieved when thebattery is correctly connected.

In other words, the potential of the source terminal of the N-channelMOSFET 37 drops lower than the gate potential of the N-channel MOSFET37, and the “H” level signal becomes applied to the gate terminal of theN-channel MOSFET 37.

The N-channel MOSFET 37 is then caused to shift to the ON state. Thesource terminal and the gate terminal of each of the N-channel MOSFETs31-1 and 31-2 included in the relay transistor circuits 21-1 to 21-3 areshort-circuited and brought to substantially the same potential, andtherefore, the N-channel MOSFETs 31-1 and 31-2 shift to the OFF state.

As a result of this operation, the relay transistor circuits 21-1 to21-3 are all switched OFF, and prevent a current from flowing from thesub-battery 12 being the high-potential side (e.g., +12 volts) into themain battery 11 being the low-potential side (e.g., −12 volts).

[3.2] Operation with Reversely Connected Sub-Battery

It is now assumed that, at an initial state, all of the N-channelMOSFETs 31-1 and 31-2 included in the relay transistor circuits 21-1 to21-3 are CFF, and the main battery 11 and the sub-battery 12 aredisconnected.

In such a configuration, because the cathode terminal of the diode D11is connected with the cathode terminal of the diode D12, as illustratedin FIG. 3, no current will flow from the sub-battery 12 being thehigh-potential side (e.g., +12 volts) into the main battery 11 being thelow-potential side (e.g., −12 volts).

The potentials of the gate terminals of the N-channel MOSFETs 31-1 and31-2 are at the ground level.

When the sub-battery 12 is reversely connected, the potentials of thesource terminals of the N-channel MOSFETs 31-1 and 31-2 are brought tothe potential equal to the sum of the drop voltage Vf of the parasiticdiode of the N-channel MOSFET 31-2 and the potential of the sub-battery12 (e.g., −12+Vf [V]).

As a result, the N-channel MOSFETs 31-1 and 31-2 shift to the ON state(closed state).

The potential of the source terminal of the N-channel MOSFET 37 isbrought to the potential equal to the sum of the drop voltage Vf of theparasitic diode of the N-channel MOSFET 31-2 and the potential of thesub-battery 12 (e.g., −12+Vf [V]).

The potential of the gate terminal of the N-channel MOSFET 37 is at thereference potential Vref (=0 volts: ground potential) due to thepresence of the pull-down resistor 38.

In other words, the potential of the source terminal of the N-channelMOSFET 37 drops lower than the gate potential of the N-channel MOSFET37, and the “H” level signal becomes applied to the gate terminal of theN-channel MOSFET 37.

The N-channel MOSFET 37 is then caused to shift to the ON state. Thesource terminal and the gate terminal of each of the N-channel MOSFETs31-1 and 31-2 included in the relay transistor circuits 21-1 to 21-3 arethen short-circuited and are brought to substantially the samepotential, and therefore, the N-channel MOSFETs 31-1 and 31-2 shift tothe OFF state.

As a result of this operation, the relay transistor circuits 21-1 to21-3 are all switched OFF, and prevent a current from flowing from themain battery 11 being the high-potential side (e.g., +12 volts) into thesub-battery 12 being the low-potential side (e.g., −12 volts).

[3.3] Operation with Main Battery and Sub-Battery Both ConnectedReversely

In this example as well, the potential of the source terminal of theN-channel MOSFET 37 is brought to the potential equal to the sum of thedrop voltage Vf of the parasitic diode in the N-channel MOSFET 31-2 andthe potential of the main battery 11 or the sub-battery 12 whicheverhaving a lower potential, in the same manner as in the examplesdescribed above. For example, when the potential of the main battery 11is −12.2 volts, and the potential of the sub-battery 12 is −12.1 volts,the potential of the source terminal of the N-channel MOSFET 37 isbrought to the potential equal to the sum of the drop voltage Vf of theparasitic diode in the N-channel MOSFET 31-2 and the potential of themain battery 11 (=−12.2 volts+Vf).

Because the potential of the gate terminal of the N-channel MOSFET 37 isat the reference potential Vref (=0 volts: ground potential) due to thepresence of the pull-down resistor 38, the potential of the sourceterminal of the N-channel MOSFET 37 becomes sufficiently lower than thegate potential of the N-channel MOSFET 37, and the “a” level signalbecomes applied to the gate terminal of the N-channel MOSFET 37.

Therefore, the N-channel MOSFET 37 shifts to the ON state, and thesource terminal and the gate terminal of each of the N-channel MOSFETs31-1 and 31-2 included in the relay transistor circuits 21-1 to 21-3 areshort-circuited and brought to substantially the same potential, andtherefore, the N-channel MOSFETs 31-1 and 31-2 shift to the OFF state.

As a result of this operation, the relay transistor circuits 21-1 to21-3 are all switched OFF, and prevent a current from flowing from thebattery being the high-potential side into the battery being thelow-potential side.

Furthermore, because the N-channel MOSFET 37 is used in the operationwith a reversely connected battery, as well as in the all-OFF operationas explained so far, the number of components can be reduced comparedwith when these operations are implemented using separate circuits.

As described above, the switch box 14 according to the embodimentprevents the main battery 11 and the sub-battery 12 from beingconnected, even when at least one of the main battery 11 and thesub-battery 12 is connected with its polarities reversed with respect tothose in the correct orientation of the battery. Because no reverseconnection current flows from the battery being the high-potential sideinto the other battery being the low-potential side, the switch box canachieve an in-vehicle power supply system in which the reverseconnection current can be prevented reliably.

The present invention is explained above based on the embodiment, butthe embodiment is merely exemplary, and it should be clear for thoseskilled in the art that various modifications of these elements and thecombinations of the elements are still possible, and are within thescope of the present invention.

For example, the main battery 11 and the sub-battery 12 are explained tobe lead storage batteries in the explanation above, but the sameconfiguration can be used even when at least one of these batteries is asecondary battery of any other type, e.g., a lithium-ion battery, aslong as they both have the same rated output voltage.

Furthermore, although the main battery 11 and the sub-battery 12 areexplained above to be provided as one secondary battery, at least one ofthese batteries may be provided as an assembled battery including aplurality of batteries that are serially or parallely connected andserving as one battery.

Furthermore, although the reference potential Vref that is the potentialof the gate terminal of the N-channel MOSFET 37 is explained above to bethe ground potential (=0 volts), the reference potential Vref does notnecessarily need to be the ground potential, and may be any otherpotential as long as the sign of the difference between the potential ofat least one of the main battery 11 and the sub-battery 12 and thereference potential Vref is reversed with respect to the sign of thedifference with the batteries correctly connected.

With the switch box according to the embodiment, even when a battery isconnected with its polarity reversed in a system in which a pair ofbatteries are connected in parallel, a reverse connection current flowfrom one of the batteries an the high-potential side to the otherbattery on the low-potential side can be prevented reliably.

Although the invention has been described with respect to specificembodiments for a complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art that fairly fall within the basic teaching herein setforth.

What is claimed is:
 1. A switch box comprising: a relay transistorcircuit configured to connect a pair of batteries in parallel; and abreaker circuit configured to break the relay transistor circuit when asign of a potential difference between a potential of at least one ofthe pair of batteries connected to the relay transistor circuit and apredetermined reference potential is reversed with respect to the signof the potential difference in which the batteries are correctlyconnected.
 2. The switch box according to claim 1, wherein the relaytransistor circuit includes a pair of N-channel MOSFETs having sourceterminals connected back to back, and the breaker circuit includes ashort-circuiting circuit that short-circuits the source terminal and agate terminal of each of the N-channel MOSFETs.
 3. The switch boxaccording to claim 2, wherein the short-circuiting circuit includes ashort-circuiting N-channel MOSFET connected serially between the sourceterminal and the gate terminal of each of the N-channel MOSFETs, andhaving a gate terminal set at the reference potential.
 4. The switch boxaccording to claim 3, further comprising: an off circuit configured toapply a high level voltage to the gate terminal of the short-circuitingN-channel MOSFET so as to switch off the N-channel MOSFETs included inthe relay transistor circuit, based on an all-off control signal fromoutside.
 5. The switch box according to claim 1, wherein the referencepotential is set to a ground potential.
 6. The switch box according toclaim 2, wherein the reference potential is set to a ground potential.7. The switch box according to claim 3, wherein the reference potentialis set to a ground potential.
 8. The switch box according to claim 4,wherein the reference potential is set to a ground potential.
 9. Theswitch box according to claim 2, wherein the pair of N-channel MOSFETshave gate terminals commonly connected, the relay transistor circuitincludes a plurality of pairs of relay transistor circuits provided inparallel between the pair of batteries, and the short-circuiting circuitis commonly connected to the N-channel MOSFETs included in each of therelay transistor circuits, and short-circuits the source terminal andthe gate terminal of each of the N-channel MOSFETs simultaneously. 10.The switch box according to claim 3, wherein the pair of N-channelMOSFETs have gate terminals commonly connected, the relay transistorcircuit includes a plurality of pairs of relay transistor circuitsprovided in parallel between the pair of batteries, and theshort-circuiting circuit is commonly connected to the N-channel MOSFETsincluded in each of the relay transistor circuits, and short-circuitsthe source terminal and the gate terminal of each of the N-channelMOSFETs simultaneously.
 11. The switch box according to claim 4, whereinthe pair of N-channel MOSFETs have gate terminals commonly connected,the relay transistor circuit includes a plurality of pairs of relaytransistor circuits provided in parallel between the pair of batteries,and the short-circuiting circuit is commonly connected to the N-channelMOSFETs included in each of the relay transistor circuits, andshort-circuits the source terminal and the gate terminal of each of theN-channel MOSFETs simultaneously.